Institute for the Dynamics of Complex Systems, University of Göttingen, Friedrich-Hund-Platz 1, 37077, Göttingen, Germany.
Department Theory and Bio-Systems, Max Planck Institute of Colloids and Interfaces, 14424, Potsdam, Germany.
Eur Phys J E Soft Matter. 2021 Mar 23;44(3):40. doi: 10.1140/epje/s10189-021-00038-5.
The swimming of bacteria provides insight into propulsion and steering under the conditions of low-Reynolds number hydrodynamics. Here we address the magnetically steered swimming of magnetotactic bacteria. We use Stokesian dynamics simulations to study the swimming of single-flagellated magnetotactic bacteria (MTB) in an external magnetic field. Our model MTB consists of a spherical cell body equipped with a magnetic dipole moment and a helical flagellum rotated by a rotary motor. The elasticity of the flagellum as well as magnetic and hydrodynamic interactions is taken into account in this model. We characterized how the swimming velocity is dependent on parameters of the model. We then studied the U-turn motion after a field reversal and found two regimes for weak and strong fields and, correspondingly, two characteristic time scales. In the two regimes, the U-turn time is dominated by the turning of the cell body and its magnetic moment or the turning of the flagellum, respectively. In the regime for weak fields, where turning is dominated by the magnetic relaxation, the U-turn time is approximately in agreement with a theoretical model based on torque balance. In the strong-field regime, strong deformations of the flagellum are observed. We further simulated the swimming of a bacterium with a magnetic moment that is inclined relative to the flagellar axis. This scenario leads to intriguing double helical trajectories that we characterize as functions of the magnetic moment inclination and the magnetic field. For small inclination angles ([Formula: see text]) and typical field strengths, the inclination of the magnetic moment has only a minor effect on the swimming of MTB in an external magnetic field. Large inclination angles result in a strong reduction in the velocity in direction of the magnetic field, consistent with recent observations that bacteria with large inclination angles use a different propulsion mechanism.
细菌的游动为低雷诺数水动力条件下的推进和转向提供了深入的了解。在这里，我们研究了磁导向的趋磁细菌的游动。我们使用斯托克斯动力学模拟来研究在外磁场中单一鞭毛趋磁细菌（MTB）的游动。我们的模型 MTB 由一个带有磁偶极矩的球形细胞体和一个由旋转电机驱动的螺旋鞭毛组成。该模型考虑了鞭毛的弹性以及磁和水动力相互作用。我们描述了游动速度如何依赖于模型的参数。然后，我们研究了磁场反转后的 U 型转弯运动，并发现了弱场和强场的两种情况，相应地也有两个特征时间尺度。在这两种情况下，U 型转弯时间分别由细胞体及其磁矩的转弯或鞭毛的转弯来主导。在弱场的情况下，转弯主要由磁弛豫来主导，U 型转弯时间与基于力矩平衡的理论模型大致相符。在强场的情况下，观察到鞭毛的强烈变形。我们进一步模拟了一个磁矩相对于鞭毛轴倾斜的细菌的游动。这种情况导致了有趣的双螺旋轨迹，我们将其作为磁矩倾斜和磁场的函数进行了特征化。对于小的倾斜角度（[公式：见文本]）和典型的场强，磁矩的倾斜对 MTB 在外部磁场中的游动只有很小的影响。较大的倾斜角度会导致沿磁场方向的速度大幅降低，这与最近的观察结果一致，即具有较大倾斜角度的细菌使用了不同的推进机制。
